1. Field of the Invention
The present invention relates to an ink tank having a valve to open and close a path through which ink is drawn out from the ink tank.
2. Description of the Related Art
A conventional ink tank of this kind is found, for example, in Japanese Patent Application Laid-open No. 2003-034041 (referred to as a “Patent Reference 1”) which discloses an ink tank having a negative pressure generation means in the form of a valve structure constructed of a valve film and a spring. In this ink tank, an ink path is formed between an ink accommodation portion and an ink supply port connected to a print head and has a negative pressure generation means of a valve structure arranged therein. Normally the valve film is pressed against a valve seat by a force of the spring to close the ink path. When, as a result of ink ejection from the print head, a negative pressure on the ink supply port side exceeds a predetermined level, the valve film is parted from the valve seat against the force of the spring to open the ink path temporarily. Then, ink is supplied from the ink accommodation portion to the ink supply port through the ink path and at the same time the negative pressure on the ink supply port side returns to below the predetermined level.
The open-close action of this valve film supplies ink under a predetermined negative pressure to the print head and thereby keeps ink in the print head at a desired negative pressure.
In a construction that uses a spring to bias the valve film, as in Patent Reference 1, there are drawbacks of an increased parts count because the spring is used and a possibility of degraded manufacturing efficiency of the ink tank because the spring needs to be assembled. Any deviation of an installation position of the spring results in variations in the force applied to the valve film, so a means to restrain the spring mounting position is also required. That is, a spring, a small part, needs to be mounted correctly while it is restrained in its mounting position. This makes the ink tank assembly work more complicated. Further, the fact that the spring is in contact with ink makes it difficult to select an appropriate material for the spring that can maintain the spring function and still does not adversely affect the property of ink. This may change in the ink composition. Further, necessitate a in the ink tank disclosed in the figure of Patent Reference 1, since the ink path is closed by urging a planar portion of the valve film against a flat portion of the valve seat by a spring, it is necessary to increase a surface precision of the valve disc, which in turn may degrade the manufacturing efficiency. Further, if, to compensate for a bad surface precision of the valve seat, the spring biasing force is increased to enhance the level of intimate contact between the valve seat and the valve film, the negative pressure on the ink supply port side generated by the ink ejection from the print head may not be able to overcome the biasing force of the spring, with the result that the valve film may fail to part from the valve seat, leaving the ink path closed.